service worker: Clarify the navigation preload network error message.

content/renderer/service_worker/service_worker_context_client.cc

Index: content/renderer/service_worker/service_worker_context_client.cc
diff --git a/content/renderer/service_worker/service_worker_context_client.cc b/content/renderer/service_worker/service_worker_context_client.cc
index 51c812a9cd41db7c787b5e4b77bf1f07a92e11ea..5f01d8b77c4175661ec92dfe74ea8bd5a2950117 100644
--- a/content/renderer/service_worker/service_worker_context_client.cc
+++ b/content/renderer/service_worker/service_worker_context_client.cc
@@ -424,8 +424,21 @@ class ServiceWorkerContextClient::NavigationPreloadRequest final
 
   void OnComplete(const ResourceRequestCompletionStatus& status) override {
     if (status.error_code != net::OK) {
+      std::string error_message;
+      if (status.error_code == net::ERR_ABORTED) {
+        error_message =
+            "The service worker navigation preload request was cancelled "
+            "before 'preloadResponse' settled. If you intend to use "
+            "'preloadResponse', use waitUntil() or respondWith() to wait for "
+            "the promise to settle.";
+      } else {
+        error_message = base::StringPrintf(
+            "The service worker navigation preload request failed with network "
+            "error code %d.",
+            status.error_code);

[horo, 2017/04/12 00:32:03]

I think we should not expose the error code for security and privacy reason.
Is there any other JS API which expose the network error code?

+      }
       // This will delete |this|.
-      ReportErrorToClient("Service Worker navigation preload network error.");
+      ReportErrorToClient(error_message);
       return;
     }
 
@@ -458,7 +471,7 @@ class ServiceWorkerContextClient::NavigationPreloadRequest final
         base::MakeUnique<WebDataConsumerHandleImpl>(std::move(body_)));
   }
 
-  void ReportErrorToClient(const char* error_message) {
+  void ReportErrorToClient(const std::string& error_message) {
     ServiceWorkerContextClient* client =
         ServiceWorkerContextClient::ThreadSpecificInstance();
     if (!client)